1. Field of the Invention
The present invention relates to magnetic resonance imaging (MRI) for internally imaging an object to be examined on the basis of a magnetic resonance phenomenon of nuclear spins of the object, particularly, to an MRI (magnetic resonance imaging) system and an MR (magnetic resonance) method capable of acquiring artery/vein visually separated images of the object without using a contrast medium.
2. Description of the Related Art
Magnetic resonance imaging is based on an imaging technique for magnetically exciting nuclear spins of an object located in a static magnetic field by applying a radio-frequency (RF) signal of a Larmor frequency and reconstructing an image from MR signals induced by the excitation.
For clinically obtaining blood flow images of the pulmonary field or abdomen of a patient by magnetic resonance imaging, MR angiography has been put in practical use, in which a contrast medium is injected into the object to highlight blood flows. However, this contrast MR angiography needs an invasive treatment to inject the contrast medium. First of all, mental and physical burdens on patients become large. Second, examination cost of the contrast MR angiography is still expensive. Third, there are some cases where a contrast medium cannot be injected into patients due to patient""s physical characteristics.
In cases the contrast medium cannot be injected or is not used, imaging techniques, such as time-of-flight (TOF) and phase contrast (PC) techniques are used alternatively.
The time-of-flight method and phase contrast techniques utilize an effect of flows such as blood flows. The effect of flows is attributed to either of two natures possessed by spins in motion. One is that spins simply move their positions due to flows, while the other results from phase shifts of transverse magnetization caused when spins move in a gradient field. The nature of the position movement is used for the TOF technique and the nature of the phase shifts is used for the phase contrast technique.
However, when the TOF technique or phase contrast technique is used for obtaining MR images of a patient""s pulmonary field or abdomen which depict flows of large vessels, such as the aorta, in their superior-inferior directions, it is required to scan slices located vertically to the flowing direction. That is, axial images should be acquired with a slice direction of those axial images set to the superior-inferior direction. Thus, in the case that two-dimensional slice imaging is performed to acquire such axial images, it is impossible to obtain an image in which blood flows are directly reflected. Three-dimensional image data spatially containing blood flows are therefore needed, but the number of slices increases which will cause an entire imaging time to be longer.
A novel MR imaging technique, known as an FBI (Fresh Blood Imaging) technique, has been proposed to overcome the foregoing inconveniences. In MR imaging on the FBI technique, an optimum time delayed from an R-wave of an ECG signal is predetermined, and ECG-synchronized MR scanning is performed at the delay time, thus well tracing a fresh and stable high-velocity blood flow ejected from the heart every appearance of the R-wave. In the FBI technique, three-dimensional scanning is additionally performed under the condition that signal intensities from parenchyma are actively suppressed by employing imaging conditions that include setting of a shorter repetition time TR (this causes the longitudinal relaxation time of parenchyma at rest to be insufficient) and applying an IR (Inversion Recovery) pulse or fat-suppression pulse (i.e., suppressing signals to be emanated from fat), thereby the blood flow being depicted. This eliminates the necessity of using a contrast medium, and blood flow images can be provided within a relatively shorter scan time.
For obtaining artery/vein visually separated blood flow images by using the FBI technique, a three-dimensional scan should be performed two times at different ECG-synchronized timings, and two sets of three-dimensional echo data acquired by the two-time three-dimensional scans or two sets of three-dimensional image data individually formed from the two sets of three-dimensional echo data should undergo weighted subtraction between the two sets of data.
In other words, even when the FBI technique is used, there remain some drawbacks that should be resolved. One drawback is that a longer scan time is still needed in total, because a three-dimensional scan should be performed two times. Another is that registration may be mistaken if the position of a patient""s body moves between two times of scans, which is apt to deteriorate quality of blood flow images which will be produced by the subtraction.
On the other hand, the foregoing TOF and phase contrast techniques are based on the effect of flows of fluid such as blood. Although depending on the characteristics of an MRI system, it is general that either of the TOF or phase contrast method depicts only blood flows whose flowing speed is 2 to 3 cm/s or more. Blood flowing slower than this speed is scarcely detected. For example, peripheral veins, lymphatic vessels, CSF (cerebrospinal fluid), pancreatic duct, and others of a patient (human being) are slower in flow speed, and their flow speeds are approximately 1 cm/s or lower in general. Additionally, there may occur influence of positional shifts due to heartbeats, it was almost impossible to detect such slower-speed fluid flows by the conventional techniques.
The present invention has been made to break through the foregoing current situations. A first object of the present invention is to, therefore, provide an MR imaging technique for producing high-quality blood flow images in a shorter scan time, without using a contrast medium.
A second object of the present invention is to provide an MR imaging technique, in addition to the above first object, which is capable of obtaining different types of blood flow images from echo data acquired by the same scanning, thus enriching pieces of information to be provided about blood flows.
A third object of the present invention is to depict such slower-speed flows as peripheral blood flows in a steady manner, with no contrast medium injected.
A fourth object of the present invention is to depict such slower-speed flows as peripheral blood flows in a shorter period of time in a steady and high-quality manner, with no contrast medium injected.
In order to accomplish the above first and second objects, by an MRI system and an MR imaging method according to one aspect of the present invention, a plurality of different cardiac time phases of an object are set, an MR imaging scan is performed to start at the thus-set plural different time phases so that a plurality of sets of echo data are acquired successively, and a blood flow image is produced from the plurality of sets of echo data.
Preferably, the plural different time phases are two time phases falling into the systole and diastole of one cardiac cycle of the object. Still preferably, in the scan, a first scan which starts at the time phase present in the systole and a second scan which starts at the time phase present in the diastole are performed by separated pulse sequences toward the same slice of the object or the same slice encoding for the object.
Still preferably, echo data or image data thereof resultant from the first scan and echo data or image data thereof resultant from the second scan are subject to mutual subtraction, thereby producing echo data or image data thereof repenting an arterial phase image. For example, the subtraction is weighted subtraction.
Further, one example of setting time phases is directed to detection of a signal indicative of the cardiac time phases of the object. A preparing MR sequence is performed on a region to be imaged at each of different times from cyclically-appearing heartbeat reference waves of the detected signal, a plurality of times in total, so that a plurality of frames of MR images are obtained. From the plurality of frames of MR images thus obtained, two cardiac time phases, i.e., two timings in a cardiac cycle are determined. For example, the signal indicative of the cardiac time phase is an ECG signal of the object and the heartbeat reference wave is R-waves of the ECG signal.
This provides higher-quality blood flow images in a shorter scan time, without injection of a contrast medium. Moreover, from echo data acquired in the same imaging, blood flow images such as an arterial phase image and a venous phase image, which are different in types, can be produced in a simple manner. It is therefore possible to enrich blood flow information that can be provided through one time of imaging.
Additionally, in order to accomplish the foregoing third and fourth objects, another aspect of the present invention provides an MRI system and an MR imaging method, in which a scan is performed for an object placed in a static magnetic field, using a pulse sequence including a readout gradient pulse. For performing this scan, a cardiac time phase of the object is set, and the readout gradient pulse is applied to the object in a manner that its applied direction is substantially parallel to a flowing direction of blood in the object. In these states, the scan is performed in synchronism with the cardiac time phase that has been set, with echo signals acquired. An image of either a blood flow or a parenchymal region influenced by the blood flow is produced from the echo signals.
Preferably, the readout gradient pulse has a main pulse to read out an echo signal and a control pulse, which is added to the main pulse, to control behaviors of phase of magnetic spins present in blood. For instance, the control pulse is a pulse to dephase or rephase magnetic spins. By way of example, the cardiac time phase to be set is two in total, one for a systole and one for a diastole. At each of the two cardiac time phases, the scan is performed, so that data consisting of two sets of echo signals are acquired.
Since an applied direction of the readout gradient pulse is almost made to agree with a flowing direction of blood and a dephasing or rephasing pulse is added to the readout gradient pulse, a slower-speed flow, such as blood flows in an inferior limb, can be depicted with accuracy, without using a contrast medium. Particularly, a high-quality image in which arteries and veins are visually separated can be depicted within a short period of time.